The present invention relates to ceramic articles, and more particularly to porous ceramic shapes, made from compositions which can be cast into molds or injection molded or extruded in desired configuration, which shapes find utility for a wide variety of purposes, including filters, kiln furniture, molds, furnace linings, insulation, and other similar products.
It is known in the art to prepare porous ceramic articles for a number of uses. For example, ceramics possessing open-celled porosity can be employed as filters for removing impurities from molten metals or for removing particulates from gaseous streams or other like filtration uses, while ceramics of open- or closed-cell porosity can find use as insulation or for any other purpose where low density structures are advantageous.
Recent interest in porous ceramics as filtering devices has developed as a consequence of increasingly stringent governmental regulations in the United States and Europe regarding maximum allowable particulates (e.g., soot) in the exhaust from automobile or truck diesel engines. A number of recent patents have suggested use of ceramics as filters for diesel particulates since ceramics generally possess excellent structural and dimensional stability under the high-temperature conditions encountered during filtration of the exhaust and subsequent burn-off (combustion) of the trapped particulates, with the requisite porosity being accomplished by various compositional and processing techniques. Most notable in these efforts has been the utilization of so-called ceramic monolithic honeycomb filtering elements as described, for example, in U.S. Pat. Nos. 4,276,071 and 4,364,761 assigned to General Motors Corp.; U.S. Pat. Nos. 4,329,162; 4,415,344; 4,416,675; 4,416,676; 4,417,908; 4,419,108 and 4,420,316 assigned to Corning Glass; and U.S. Pat. Nos. 4,283,210; 4,293,357; 4,307,198; 4,340,403; and 4,364,760 assigned to NGK Insulators. Essentially, these elements consist of a monolithic ceramic having a multitude of internal parallel chambers separated by thin porous ceramic internal walls, with a number of the chambers being end-sealed so as to force particulate-containing exhaust gas to traverse across a porous wall before exiting the element. Generally, these elements are formed by an extrusion process, and materials are included in the compositions which are burned out during the firing process so as to provide the requisite porosity in the internal filtering surfaces. In another process, as reflected in U.S. Pat. No. 4,363,644 assigned to Nippon Soken, foamed, structural polyurethane systems are utilized in admixture with inorganic materials in processes wherein the polyurethane burns out during firing so as to leave behind a ceramic structure having a variety of interconnected open cells for trapping particulates.
While the structural and dimensional properties of ceramics generally lend themselves well to utilization as the material from which filter elements for diesel traps are constructed, it is not an easy or inexpensive matter to achieve from ceramic materials elements possessing the porosity needed to effectively and efficiently filter soot as well as permit exhaust gas flow without substantial build-up of back pressure. Additionally, efforts toward optimizing the geometry of diesel filter trap designs (so as to facilitate inclusion of the filter in the exhaust area of a vehicle, or to maximize filtration, or to facilitate regeneration or removal of the filter element) can be severely limited by the inability to produce such shapes efficiently (or at all) utilizing ceramics.
These same problems, i.e., difficulty in economic attainment of desired porosity and lack of design flexibility in produceable shapes or configurations, are also present with respect to many other desired uses of porous ceramics besides particulate filters, and constitute significant limitations on the use of porous ceramics for such other possible uses. For example, many filtering applications which require use of filter aids (e.g. diatomaceous earth) could in theory be economized significantly by provision of a porous ceramic as the essential filtering element (i.e., eliminating the need for use of filter aids which are disposed of after each filtration), but efforts in this regard have been hampered by the inability to produce such porous ceramics economically and with sufficient control over porosity and other characteristics. Additionally, strong impetus exists for producing porous ceramics for low-mass applications which do not rely upon potentially hazardous inorganic fibers to provide porosity, but again efforts in this regard have been plagued by lack of economically feasible compositions and processes for making such porous ceramics.
The present invention addresses and overcomes these problems by provision of compositions and processes which enable the manufacture of porous ceramics of any desired size and shape as well as any desired form or extent of porosity. As a consequence, the range of potential uses for porous ceramics is greatly extended.